Implantable medical devices (IMDs) such as pacemakers and implantable cardiovertor defibrillators (ICDs) typically include sensors for monitoring physiological signals from a patient, such as cardiac electrodes for sensing cardiac electrogram (EGM) signals. Using the sensed signals, the IMD may be configured to compute measurements corresponding to a patient's cardiac rhythm and use that information in computing therapy parameters controlling an automatically delivered therapy, such as a cardiac pacing therapy, which may be a single chamber, dual chamber or multi-chamber cardiac resynchronization therapy (CRT), anti-tachycardia pacing (ATP) or other pacing therapies.
In some examples, control parameters determined from a sensed signal may be computed from measurements of a physiological variable taken over a relatively short period of time, such as one cardiac cycle or several cardiac cycles. In other examples, a parameter may be desired which requires computation from a physiological variable that requires measurements over a relatively longer period of time, such as one minute, several minutes, hours, weeks or more. A relatively large amount of IMD memory is required for storing signal data as it is acquired for use in computing of a physiological variable using long-term measurements of a physiological signal. Computing a time varying physiological variable from signal measurements may require complex computational methods, such as differential, exponential, logarithmic or other non-linear functions, which require generally high processing power and time. Computation of therapy control parameters using measurements obtained over long time periods and/or requiring high computational complexity can be limited in an IMD because of the overall size limitations, which limits battery size, memory size, and processing power. A need remains, therefore, for IMD systems and associated methods that provide efficient computation of an estimated or predicted value of a physiological variable for use in setting a therapy control parameter and/or for assessing a patient condition.